Medical ultrasound imaging is a widely used and cost effective diagnostic tool in internal medicine. In essence, ultrasound imaging systems use sound waves to create an image of a patient's internal tissues and organs. A physician can the use this image as a diagnostic tool in treating the patient. Structurally, an ultrasound imaging system typically includes a hand-held handle unit, a main system unit physically separated from the handle unit, and coaxial cables connecting the handle unit to the main system unit. The handle unit contains a transducer or transducer array and may include preamplifiers. The system unit contains additional processing circuits to produce the ultrasound images.
To operate the imaging system, a medical operator places the handle unit at a selected position adjacent to a patient's body. The operator turns on the imaging device and electrical transmission pulses are sent to the transducer. The transducer converts these electrical transmission pulses into sound waves which are directed into the patient's body. These sound waves penetrate the patient's body and components of the sound waves are reflected back toward the handle unit and impact the transducer. The transducer converts the acoustical energy from the reflected sound waves into electrical input signals representing the reflected sound waves. These input signals are transferred from the handle unit through the coaxial cable to the system box where the input signals are processed to produce diagnostic output readings. The input signals may be preamplified in the handle unit prior to transmission across the coaxial cables, but most current systems do not include preamplifiers in the handle unit.
For the typical medical ultrasound system, the transducer in the handle unit includes arrays of piezoelectric transducers. Ultrasound imaging systems produce improved images if the system is operated at high frequencies, but to produce these high frequencies large numbers of small element are necessary. Use of large numbers of small elements at the remote sensor creates a high effective source impedance which causes much of the input signal to be lost when the input signal is transmitted across high capacitance coaxial cables.
This problem of high source impedance and input signal loss during transmission across the coaxial cables is sometimes handled by the prior art by placing a preamplification stage in the handle unit. The preamplification stage typically uses JFET or MOSFET devices in a common source architecture. Resistive biasing means are used to bias the transistors used in the handle unit preamplifiers. One problem with this method of preamplification is that the preamplifier load resistor must be small to attain an acceptable bandwidth for the preamplifier. The small load resistor, however, reduces the preamplifier gain and causes greater noise. Another problem is that the resistive biasing of the preamplifier transistors creates additional noise in the handle unit preamplifier. Placing a complete preamplifier in the handle also leads to heat dissipation problems and possible bulkiness of the handle unit.
In some ultrasound system applications, the transducer signals are multiplexed. Multiplexing involves a system design that permits the system unit to select only a portion of the transducers to be read out and processed for each transmission pulse. When using a multiplexing system, switching must be provided in the handle to allow for selective operation of only a portion of the transducers.